How Refrigerated Shipping Containers (Reefers) Work

Reefer is the industry's shorthand for a refrigerated shipping container: a steel shipping box carrying an integrated cooling unit and insulated walls, built the same way a standard box is built. The set point it holds can run anywhere from deep-frozen to just above ambient, whether the unit is crossing an ocean or sitting on your property as cold storage. Buyers weighing a reefer for a farm, a restaurant, a lab, or a distribution yard benefit from knowing what the machinery behind that steel door is actually doing.

Ahead: how the cooling unit and insulation keep cargo cold, the temperature range a reefer can hold, what it needs for power and a genset, the most common cold and frozen storage jobs, the difference between a new unit and a refurbished one, and what shapes running cost. Treat the figures as general industry guidance rather than a quote — the specifics come down to your unit, your climate, and what you're storing.

Short version

Think of a reefer as a precision walk-in freezer built into the shell of a shipping container. On the front wall, an integrated refrigeration unit draws heat out of the box and drives chilled air through the cargo, and the insulated walls, floor, and doors slow that heat from working its way back in. A thermostat locks in the set point, commonly somewhere between roughly -20degF and 70degF depending on the model, and the unit is powered by 3-phase electrical power — a grid hookup where one exists, or a diesel generator (genset) where it doesn't.

Inside the cooling unit and insulation

The refrigeration machinery lives in the front wall as one integrated unit, and it runs a standard refrigeration cycle to do its job: a compressor pressurizes the refrigerant, a condenser sheds heat outside the box, and an evaporator pulls heat out of the air inside. What comes out of that cycle is a stream of chilled air that the unit pushes through the container.

That chilled air isn't just blown loosely across the load. A reefer's floor is a T-shaped aluminum grating (called a T-floor) built so cold air can run the length of the container beneath the cargo, rise up through and around it, then return to the unit. Delivering air from the bottom this way keeps the temperature consistent from the doors to the front wall, and it's the reason cargo goes in loaded for airflow rather than packed solid to the ceiling.

Insulation is what makes holding that temperature practical. Foam insulation fills the walls, roof, floor, and doors of a reefer, and the door seals (gaskets) are built heavier than the ones on a dry container. Wall insulation on a standard reefer runs roughly 4 to 5 inches thick, which is why the interior comes out slightly narrower and shorter than a same-size standard dry container, the general-purpose steel container for dry cargo. You give up that space in exchange for holding temperature: the tighter the seal and the thicker the insulation, the less the cooling unit has to work.

Airflow decides more than raw cooling power

What a reefer actually maintains is air temperature, not product temperature. It's designed to hold cargo that already arrives at the right temperature, not to pull a warm load down quickly. Load it warm, block the floor channels, or leave the doors open often in summer heat, and the unit has to work harder while the temperature drifts. Get the loading and door discipline right, though, and it holds its set point without trouble.

How wide is the temperature range a reefer can hold?

Most reefer units cover a wide band of set points, commonly running from roughly -20degF at the frozen end up to around 70degF for produce or controlled ambient storage. That range handles the great majority of cold and frozen use cases, everything from ice cream to fresh greens.

Specialized units go further still. Deep-freeze and "super-freezer" reefers, built for products like seafood and pharmaceuticals, can run well below -20degF, and units fitted with controlled-atmosphere or humidity controls add extra protection for sensitive produce. Any specific low-temperature need is really a spec question: the exact floor changes by make and model, so confirm it against your contents instead of assuming every reefer bottoms out at the same point.

How much power, and what kind of generator, does a reefer need?

The cooling unit on a reefer runs on 3-phase electrical power. For a fixed site, that typically means a dedicated 3-phase hookup put in by a licensed electrician, since single-phase household power generally can't run the unit the way it's designed to run. It's the single biggest difference in site prep between a reefer and a standard dry container, which needs no power hookup at all.

Wherever a suitable hookup doesn't exist (a remote farm, an event, a job site, or a unit still in transit), the reefer instead runs off a generator, called a genset, sized to match its draw. Gensets come as clip-on units that attach directly to the container or as standalone trailer-mounted generators. Choosing that route means budgeting for fuel and routine generator maintenance in addition to the container itself.

The unit has to run continuously to hold its temperature, so power is an ongoing operating requirement rather than something you set up once and forget. If the cargo is perishable, having a backup plan for outages matters.

Where cold and frozen storage gets used

A reefer pays for itself anywhere temperature-controlled space is needed without building a permanent cold room. The common jobs:

• Deep-frozen storage— spare capacity at deep-freeze set points for food distributors handling seafood, meat, and ice cream.
• Chilled-goods storage— used to keep produce, dairy, beverages, and floral stock just above freezing.
• Grocery and restaurant overflow— cold and frozen space added during peak season or while remodeling.
• Agricultural and farm use— used on-site to cool harvested produce before it ships out.
• Lab and pharmaceutical storage— for temperature-sensitive materials that need tight, monitored control.
• Disaster response and events— portable refrigeration for sites without a fixed cold room, usually run on a genset.

What makes a reefer useful is exactly why buyers pick it over a built cold room: it holds temperature anywhere you can power it, it's mobile and self-contained, and it can be relocated or handed back once the need is over.

Choosing between a new (one-trip) and refurbished reefer

Buyers choose between two practical reefer grades. One option, the one-trip unit, made a single loaded voyage from the factory overseas before it was sold (about as close to new as a container gets), and it comes with the freshest cooling unit, the longest remaining service life, and the top price tag. The other option, a refurbished reefer, has been professionally repaired, repainted, and serviced to working condition, including a cooling unit that has been serviced and tested, and it costs less while still suiting most stationary cold and frozen storage, since the unit stays put instead of crossing an ocean.

Age by itself isn't the deciding question; the condition of the cooling unit is. On any used reefer the machinery counts for more than the paint, so a sound, recently serviced unit on an older box beats a tired unit sitting in a clean-looking shell. Regardless of grade, every container we sell is inspected and graded before delivery, and on a reefer that inspection covers confirming the cooling unit runs and actually holds temperature.

For a side-by-side look at reefer sizes next to standard dry or high cube options in the same footprint, our guide to shipping container sizes sets the interior dimensions next to each other, making clear exactly how much usable space the insulation takes from you.

What determines a reefer's running cost?

The purchase price of a reefer is only part of the picture — the operating factors below decide what it actually costs to run.

• Electrical draw— the unit runs around the clock, so electricity (or genset fuel) makes up the largest ongoing expense, and a grid hookup is typically cheaper to run over time than a diesel genset.
• Target temperature— a deep-freeze set point demands more energy than chilling just above freezing, which is why frozen storage runs more expensive than chilled.
• Climate and sun exposure— a unit sitting in direct sun in a hot climate cycles harder than one kept in shade or a milder region, and shade plus good airflow around the box both help.
• Condition of the insulation— heat gets in through worn door gaskets or damaged insulation and forces the unit to run harder, which is exactly why the inspection step matters.
• Servicing— the cooling unit, and any genset, needs routine service to stay efficient and to avoid a failure that puts cargo at risk.

There's no running-cost figure published here, because the real number depends entirely on your set point, climate, contents, and power source. Those inputs are what decide it, and a site review is how you pin the figure down.

Common questions

Can a reefer freeze cargo, or only chill it?

Both. Most units cover a wide band of set points, commonly from roughly -20degF up to about 70degF, so the same box can run as a freezer or a chiller depending on what you set it to. Deep-freeze models reach lower still — confirm the floor against your specific contents.

Does a refrigerated container need special power?

In most cases, yes. The cooling unit runs on 3-phase electrical power, which on a fixed site means a dedicated hookup put in by a licensed electrician. Where there's no hookup, a generator (genset) sized to the unit's draw runs it instead.

If the power goes out, how long does a reefer hold temperature?

Insulation buys some time, but a reefer holds temperature by running continuously rather than by storing up cold. If the cargo is perishable, plan on a backup power source instead of counting on the box to coast through an outage.

Does a refurbished reefer hold up for storage use?

Yes, for stationary cold and frozen storage, as long as the cooling unit has been serviced and tested first. What matters most on a used reefer is the machinery, not the paint job, so before delivery the unit gets checked to confirm it actually holds temperature.

If a reefer suits your project, the 20ft reefer unit comes as a refurbished unit, its cooling unit serviced, tested, and working, quoted against your site and contents; typical delivery for a reefer runs 7–10 business days. Take a look through the full refrigerated container range to see what's available, and when you contact us, let us know your set point, location, and power setup. Expect a reply within 1 business day.